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In the cumene process, which converts cumene to phenol and acetone, there is a step where cumene hydroperoxide undergoes acid-catalyzed rearrangement:

A reaction scheme showing the rearrangement of cumene hydroperoxide in acid

(Image source: Wikipedia)

Why does the phenyl shift happen preferentially? The methyl group is smaller and should have a higher migratory aptitude.

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Despite the mechanism shown in your question, which is commonly used to depict a phenyl shift, phenyl shifts occur by a slightly different mechanism.

A scheme showing the mechanism of cumene hydroperoxide rearrangement through formation of a resonance-stabilized spriobicyclic cation

Phenyl shifts do not usually happen by the normal 1,2-sigmatropic "hop". Instead, the phenyl group shifts by more of a "walk". It's not really a sigmatropic rearrangement. First, one of the $\pi$-bonds in the phenyl group can attack the electrophilic center. This leads to formation of a spirobicyclic intermediate where the cation is delocalized throughout the rest of the ring. Then, the three-member ring breaks open at the expense of a carbon-carbon bond to rearomatize the phenyl ring and form the new resonance stabilized cation.

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  • $\begingroup$ That's pretty neat. Is this mode assumed based on kinetics from ring substitution? $\endgroup$ – Zhe Jan 11 '17 at 13:07
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    $\begingroup$ I wonder if there is a name for the intermediate carbocation. I recall the carbon analogue is called a phenonium ion and can arise via similar chemistry, however the IUPAC definition strictly disqualifies this one with an oxygen in it. $\endgroup$ – orthocresol Jan 11 '17 at 19:37
  • $\begingroup$ Hi, thanks for your answer! If I may ask, could you please tell how do we know the phenyl shift occurs by a "walk" and not a "hop"? (nice analogy btw!) Do we have experimental evidences, or some other proof? Many thanks! $\endgroup$ – Gaurang Tandon Mar 17 '18 at 16:00

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